The present invention relates to a semiconductor device having a backplate electrode for obtaining a uniform backplate potential of a semiconductor pellet.
Generally, when a semiconductor pellet integrating a number of circuit elements therein is mounted or attached to its frame, the electrical connections between circuit elements and input/output terminals or between the circuit elements and the circuit ground, a heat dissipating property from the pellet to the frame, and the mount strength between the pellet and the frame must be taken into consideration. The mount strength must be highly reliable for a long period of time.
In order to stabilize operations of circuits integrated in the pellet, a metal film is normally applied on a backplate of the semiconductor pellet to cause a substrate of the pellet to have the same potential as a whole. The metal film has, e.g., a three-layer structure, as shown in FIG. 1. That is, vanadium (V) substrate contacting layer 12 is formed on silicon (Si) substrate 11, nickel (Ni) interlayer 13 is formed on V layer 12, and gold (Au) surface layer 14 is formed on Ni interlayer 13. Ni interlayer 13, i.e., a second layer of the above structure, is used as a backplate-electrode main body for obtaining a uniform backplate potential of the semiconductor pellet. V layer 12, i.e., a first layer of the structure, serves to improve an electrical characteristic of the pellet, particularly to reduce the contact resistance between Ni interlayer 13 and Si substrate 11 and to increase a physical adhesive strength between interlayer 13 and substrate 11. Third layer 14 is provided to prevent oxidation of Ni interlayer 13 and is often made of an Au film (or an Au alloy film) having an excellent moisture-proof property.
In order to mount the semiconductor pellet having the above structure to a frame, when a main circuit element in the pellet is a power element which generates a large amount of heat, a solder-mount method or a eutectic-mount method is used. On the other hand, when a circuit in the pellet is constituted by elements which generate less heat, a simple mount method using a conductive resin can be utilized.
In FIG. 1, if solder mounting is performed, using Ni interlayer 13 as a backplate surface layer of the semiconductor pellet, without coating Ni interlayer 13 with Au layer 14, an oxide film is formed on the surface of Ni interlayer 13, causing poor conductivity. On the other hand, even when the structure of FIG. 1, whose backplate surface layer is coated with Au layer 14, is used, Au layer 14 having a film thickness of 2,000 .ANG. or more is needed to obtain a perfect oxidation-preventing effect, because a thin Au film is porous. In addition, although Au is superior in moisture resistance, it tends to react with other materials to pose problems. For example, Au reacts with Sn in solder to form an Au-Sn alloy. However, because of its high eutectic reaction speed, the Au-Sn alloy tends to be a dispersion-type alloy of a brittle .beta. phase instead of one having a uniform (.alpha.+.beta.) eutectic texture For this reason, the Au film used during solder mounting lacks reliability in terms of physical strength. In addition, when the Au film reacts with the Si substrate, electrical resistance of the Au film is increased. Furthermore, when a semiconductor pellet having no Au film at its backplate surface is mounted on the frame by a conductive resin, defects tend to occur due to oxidation of the Ni film. Even when the backplate surface of the pellet is coated by an Au film, if its thickness is less than 2000 .ANG., since such a thin Au film often involves pinholes and is porous as has been mentioned, the resultant goods are liable to be defective. This is because an acid or alkaline solvent decomposed from the resin accelerates oxidation of the Ni film at a higher temperature and higher humidity.
As described above, the backplate electrode surface layer (a) must not be easily oxidized, (b) must not easily react with other materials (elements), and (c) must have a good conductivity. In addition, in terms of device cost, it is desired that the backplate electrode surface layer (d) can be manufactured using an inexpensive material, rather than an expensive noble metal such as Au.